The present invention relates to a method and an apparatus for image formation and more particularly to a method and an apparatus for image formation using a ferroelectric liquid crystal element or device.
Liquid crystal devices have heretofore been used in the fields of display, optical shutters and the like because they provide apparatus in a small size, in a thin form and with a low power consumption. Especially, in the field of display, there have been a rapid progress based on several noticeable inventions. For example, the application of TN (twisted nematic) liquid crystals have been known, as described in "Voltage-Dependent Optical Activity of a Twisted Nematic Liquid Crystal" by M. Schadt and W. Helfrich, Applied Physics Letters, Vol. 18, No. 4 (Feb. 15, 1971), pp. 127-128. Liquid crystal devices used in the field of display generally have an X-Y matrix electrode structure in order to arrange image display elements (picture elements) in the form of a matrix.
As a method for driving such a display device, there have been generally adopted a time-sharing driving method wherein address signals are sequentially, periodically and selectively applied to scanning electrodes, and predetermined information signals are applied to signal electrodes. However, when the number of picture elements is increased in the display device or the driving method, the duty ratio is decreased, thus resulting in lowering of image contrast and occurrence of cross-talk. Moreover, in order to reduce the size of picture elements and obtain an improved resolution, it is necessary to arrange the matrix electrodes and connect them with lead wires at a high packaging density, whereby manufacture of the devices become complicated.
Another display system is disclosed, e.g., by T. P. Brody, Juris A. Asars and G. Douglas Dixon, IEEE Transactions on Electron Devices, Vol. ED-20, (No. 11, Nov. 1973), pp. 995-1001, "A 6.times.6 Inch 20 Lines-per-Inch Liquid Crystal Display Panel", wherein respective picture elements are provided with a thin film transistor (TFT) and switched one by one. In this system, however, the provision of TFT to respective picture elements is complicated, and considerable improvement is desired in respect to production cost.
On the other hand, as a system for applying image signals to a liquid crystal device, a heat-scanning system by means of, for example, laser with long wave-lengths has been known, in addition to the one employing a matrix electrode structure. This system employing heat-scanning for applying image signals has an advantage of requiring a much smaller number of lead wires because it does not necessitate the high-density matrix electrode structure. This heat-scanning system for writing image signals in a liquid crystal device is illustrated, for example, in FIG. 1.
In FIG. 1, reference numeral 11 denotes transparent base plates such as glass plates, 12 a transparent electroconductive layer such as of ITO (Indium Tin Oxide), 13 an aluminum reflection film, and 14 an orientation controlling film for controlling orientation of a liquid crystal. Reference numeral 15 denotes a layer of a liquid crystal which causes phasetransition of smectic phase.fwdarw.numatic phase.fwdarw.isotropic phase according as temperature increases. The thickness of the liquid crystal layer 15 is held constant by a spacer 10. Further, liquid crystal molecules constituting the liquid crystal layer 15 are ordinarily uniformly aligned vertically (homeotropically) or horizontally (homogeneously) with respect to the cell face due to wall effect of the orientation controlling film 14.
The operation of the liquid crystal device is explained hereinbelow. At the time of writing images, the liquid crystal layer 15 is held at a smectic side temperature close to the smectic-nematic transition temperature. Then, the liquid crystal layer 15 is irradiated with imagewise laser beam 17A from, e.g., YAG laser and only the irradiated image portion 19 of the liquid crystal layer 15 is transformed into the nematic or isotropic phase. When the irradiation of laser beam is removed, the transformed portion is rapidly cooled and transformed into a light-scattering smectic phase 19. When a readout light flux 17 is irradiated to a cell having such a scattering smectic phase portion 19 from the side of the transparent electrode 14, light beams 17a and 17c are reflected by the aluminum reflection film 13 into an almost constant direction, whereas light beam 17b irradiated to the scattering portion 17 is scattered. Accordingly, when the readout light 17 is projected through the cell onto a screen 16, only light beam 17a and 17c are projected onto the screen but light beam 17b is hardly projected. Thus, the image recorded in the liquid crystal layer is projected as such on the screen 16.
The image recorded in the liquid crystal layer 15 can be erased by applying a voltage to the all from an AC power supply 18 or by heating the whole cell into the nematic or isotropic phase and then gradually cooling the cell. While such a liquid crystal display device can provide a large area of display with a memory characteristic at a high density, it requires a large output laser and a long time for writing one picture of image. Moreover, if the area of the liquid crystal device is increased, the writing time is further increased. Accordingly, the system explained with reference to FIG. 1 can only be applied to a display apparatus of an enlarged projection type.
As another method using no matrix electrode, there is known one using an electron beam for writing. This method is however accompanied with such defects, as in a method using CRT, that a high resolution cannot be obtained because of spreading of the electron beam and that the apparatus requires a large length behind the display face.